The present invention relates generally to the use of polymer composite materials for the protection of electronic components against electrical overstress (EOS) transients.
There is an increased demand for electrical components which can protect electronic circuits from EOS transients which produce high electric fields and usually high peak powers capable of destroying circuits or the highly sensitive electrical components in the circuits, rendering the circuits and the components non-functional, either temporarily or permanently. The EOS transient can include transient voltage or current conditions capable of interrupting circuit operation or destroying the circuit outright. Particularly, EOS transients may arise, for example, from an electromagnetic pulse, an electrostatic discharge, lightening, or be induced by the operation of other electronic or electrical components. Such transients may rise to their maximum amplitudes in microsecond to subnanosecond time frame, or less, and may be repetitive in nature. A typical waveform of an electrical overstress transient is illustrated in FIG. 1. The peak amplitude of the electrostatic discharge (ESD) transient wave may exceed 25,000 volts with currents of more than 100 amperes. There exist several standards which define a simulation waveform of the EOS transient. These include IEC 1000-4-2, ANSI guidelines on ESD (ANSI C63.16), DO-160, and FAA-20-136. There also exist military standards, such as MIL STD 461/461 and MIL STD 883 part 3015.
Materials for the protection against EOS transients (EOS materials) are designed to respond essentially instantaneously (i.e., ideally before the transient wave reaches its peak) to reduce the transmitted voltage to a much lower value and clamp the voltage at the lower value for the duration of the EOS transient. EOS materials are characterized by high electrical resistance values at low or normal operating voltages and currents. In response to an EOS transient, the material switches essentially instantaneously to a low electrical resistance value. When the EOS threat has been mitigated these materials return to their high resistance value. These materials are capable of repeated switching between the high and low resistance states, allowing circuit protection against multiple EOS events. EOS materials are also capable of recovering essentially instantaneously to their original high resistance value upon termination of the EOS transient. For purposes of this application, the high resistance state will be referred to as the xe2x80x9coff-statexe2x80x9d and the low resistance state will be referred to as the xe2x80x9con-state.xe2x80x9d This transition between resistance states is not a step function, instead transitioning between the off-state and the on-state in a non-linear manner. These materials which are subject of the claims herein have withstood thousands of ESD events and recovered to desired off-states after providing protection from each of the individual ESD events.
FIG. 2 illustrates a typical electrical resistance versus d.c. voltage relationship for EOS materials. Circuit components including EOS materials can shunt a portion of the excessive voltage or current due to the EOS transient to ground, thus, protecting the electrical circuit and its components. The major portion of the threat transient is reflected back towards the source of the threat. The reflected waive is either attenuated by the source, radiated away, or re-directed back to the surge protection device which responds with each return pulse until the threat energy is reduced to safe levels.
U.S. Pat. No. 2,273,704, issued to Grisdale, discloses granular composites which exhibit non-linear current voltage relationships. These mixtures are comprised of granules of conductive and semiconductive granules that are coated with a thin insulative layer and are compressed and bonded together to provide a coherent body.
U.S. Pat. No. 2,796,505, issued to Bocciarelli, discloses a non-linear voltage regulating element. The element is comprised of conductor particles having insulative oxide surface coatings that are bound in a matrix. The particles are irregular in shape and make point contact with one another.
U.S. Pat. No. 4,726,991, issued to Hyatt et al., discloses an EOS protection material comprised of a mixture of conductive and semiconductive particles, all of whose surfaces are coated with an insulative oxide film. These particles are bound together in an insulative binder. The coated particles are preferably in point contact with each other and conduct preferentially in a quantum mechanical tunneling mode.
U.S. Pat. No. 5,476,714, issued to Hyatt, discloses EOS composite materials comprised of mixtures of conductor and semiconductor particles in the 10 to 100 micron range with a minimum proportion of 100 angstrom range insulative particles, bonded together in a insulative binder. This invention includes a grading of particle sizes such that the composition causes the particles to take a preferential relationship to each other.
U.S. Pat. No. 5,260,848, issued to Childers, discloses foldback switching materials which provide protection from transient overvoltages. These materials are comprised of mixtures of conductive particles in the 10 to 200 micron range. Semiconductor and insulative particles are also used in this invention. The spacing between conductive particles is at least 1000 angstroms.
Examples of prior EOS polymer composite materials are also disclosed in U.S. Pat. Nos. 4,331,948, 4,726,991, 4,977,357, 4,992,333, 5,142,263, 5,189,387, 5,294,374, 5,476,714, and 5,669,381.
None of these prior patents disclose an EOS composition comprising a doped semiconductor. Further, it has yet to be recognized that the switching characteristics of an EOS composition can be controlled by varying the level of doping of a semiconductor. The present invention meets these and other needs.
In a general aspect of the present invention there is provided polymer composite materials which exhibit a high electrical resistance to normal operating voltage values, but in response to an EOS transient switch to a low electrical resistance and clamp the EOS transient voltage to a low level for the duration of the EOS transient.
In a first embodiment of the present invention the EOS composition comprises an insulating binder, doped semiconductive particles, and semiconductive particles.
In a second embodiment of the present invention the EOS composition comprises an insulating binder, semiconductive particles doped to have a first electrical conductivity, and semiconductive particles doped to have a second electrical conductivity.
In a third embodiment of the present invention the EOS composition comprises an insulating binder, conductive particles composed of an inner core and an outer shell, and semiconductive particles. The inner core of the conductive particles comprises an electrically insulating material and the outer shell comprises one of the following materials: (i) a conductor; (ii) a semiconductor; (iii) a doped semiconductor; or (iv) an insulating material other than the material comprising the inner core. Alternatively, the inner core of the conductive particle may comprise a semiconductive material and the outer shell comprise one of the following materials: (i) a conductor; (ii) a semiconductive material other than the material comprising the inner core; or (iii) a doped semiconductor. In yet a further alternative embodiment wherein the conductive particles are comprised of a core-shell structure, the inner core is comprised of a conductive material and the outer shell is comprised of one of the following materials: (i) a conductive material other than the material comprising the inner core; (ii) a semiconductor; or (iii) a doped semiconductor.
In a fourth embodiment of the present invention the EOS composition comprises an insulating binder, conductive particles composed of an inner core and an outer shell, and doped semiconductive particles. The materials of the core-shell structured conductive particles may include any one of the combinations set forth above with respect to the third embodiment of the present invention.
Finally, each embodiment of the present invention may optionally include small amounts of insulative particles.
Other advantages and aspects of the present invention will become apparent upon reading the following description of the drawings and detailed description of the invention.